The present invention relates to axial magnetic field electric machines, such as motors, generators, tachogenerators, and the like, and, more particularly, to a axial magnetic field electric machines employing permanent magnet rotor.
Electric machines employing permanent magnets have been known for many years. Recently considerable progress has been made in such machines with the increased quality and reduced prices of rare-earth metal magnets, such as Sm-Co, Nd-Fe-B, and the like. Such machines are currently available which are capable of reaching the low torque range as well as several tens of Newtonometers.
Applications with more rigorous space-saving requirements, such as in robotics, machine-tools, electric automobiles, and the like, generally require disc-shaped machines, especially axial air-gap motor and generators.
Various configurations of axial air-gap machines are currently known, including machines with separated permanent magnets on the rotor, with a single piece magnetized rotor, with toothed and slide cores, single or multiple rotor and stator design, with sector or toroidal windings, and the like.
Some of these configurations are disclosed in U.S. Pat. Nos. 4,835,840 (to Stokes), 5,117,141 (to Hawsey et al.) and 4,996,457 (to Hawsey et al).
U.S. Pat. No. 4,835,840 describes disc rotor assembly which includes a ring of magnetic flux conducting material having a ring-shaped magnetizable body fastened thereon. The device includes a flat, axially facing, ring-shaped rotor surface for opposing a single stator. It is to be noted that such an assembly cannot be used in the more preferable two stator "sandwich" design and cannot utilize widely distributed flat magnet pieces.
U.S. Pat. No. 5,117,141 discloses several rotors for brushless DC motors. Each of these includes one or more sets of permanent magnets arranged in a ring configuration around the peripheral edge. The magnets are imbedded directly in a carrying disc and connected to each other without an intermediate structural element, such as a ring, a sheet, and the like. Besides, the above-referenced patent states that such rotors are to be used in drive units which cause rotation of two shafts in opposite directions, each shaft being connected to its rotor.
A disadvantage of such axial gap permanent magnet electrical machines, especially at the high rotational speeds, is that the rotor has a tendency to fail mechanically. The underlying reason for these failures is that rotors operate under strong cyclic force fields while the need to axially conduct magnetic flux makes it difficult to design a rigid carrying structure. Because such rotors tend to fail mechanically, requiring costly equipment downtime and expensive repair or replacement.
There is thus a widely recognized need for, and it would be highly advantageous to have, an axial gap electric machine employing permanent magnet rotors of rigid and firm design which would be more mechanically rugged and operationally reliable.